Cu-CONTAINING LOW ALLOY STEEL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE, AND METHOD FOR MANUFACTURING THE SAME

ABSTRACT

A Cu-containing low alloy steel includes a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050% and Nb: 0.020 to 0.080% and N: 0.005 to 0.020%, and as required, Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities.

TECHNICAL FIELD

The present invention relates to a Cu-containing low alloy steel suitable as a steel for offshore structures to be used for mooring facilities, risers, flowlines and the like, and a method for manufacturing the same.

BACKGROUND ART

Petroleum and natural gas are broadly used as the core of energy. In recent years, the exploitation thereof is being shifted from the land to the sea, and particularly with regard to the sea resource exploitation, mining in deep water rather than continental shelves is becoming the mainstream. For steels for offshore structures to be used in the ultradeep-water exploitation, from the viewpoint of securing safety, not only the demand on the toughness of members themselves but also the demand on the toughness of HAZ is increased in severity, and the CTOD value has been demanded. Further from the viewpoint of the welding efficiency at worksites, since there is demanded the welding execution in the range of welding heat input of 2.0 to 4.0 kJ/mm, it is needed to satisfy a low-temperature toughness required by multi-layer welds of thick-wall forged steel products for offshore structures at a heat input of 4.0 kJ/mm or lower.

As steels for offshore structures, there is known, for example, a steel containing 0.43% by mass or lower of Cu, specified by ASTM A707. The steel is one in which by precipitating Cu by aging treatment, the strength is secured due to a low-carbon and low-carbon equivalent composition in consideration of weldability, and the strength and the low-temperature toughness are simultaneously satisfied. However, the above steel has a low Cu content and in the case of adopting two phase region quenching, cannot secure the strength of members even when having been subjected to aging treatment.

As a conventional technology, Patent Literature 1 proposes a method of improving the strength-toughness balance of a thick-wall forged steel which is used as a steel for offshore structures, in order to utilize two phase region quenching treatment and secure a low-temperature toughness by age hardening according to Cu.

Then, there are conventionally many of proposes of improving the CTOD characteristic of HAZ, and for example, in Patent Literature 2, the CTOD characteristic of HAZ is improved by a manufacturing method in which center segregation is suppressed, and by restriction of components.

Furthermore, formation of a fine structure of HAZ is achieved, in Patent Literature 3, by utilizing a complex precipitate of fine Ti nitride and MnS, and in Patent Literature 4, by utilizing fine Ti oxide or Mg oxide.

In Patent Literature 5, the suppression of island martensite (MA) is achieved by a value of M1* and a value of M2* based on the chemical composition, and the improvement of the CTOD characteristic is achieved with the thickness being 1 inch or larger and the tensile strength being 700 MPa.

CITATION LIST Patent Literature

-   [Patent Literature 1] -   Japanese Patent Laid-open No. 2017-150041 -   [Patent Literature 2] -   Japanese Patent Laid-open No. 09-001303 -   [Patent Literature 3] -   Japanese Patent Laid-open No. 05-099619 -   [Patent Literature 4] -   Japanese Patent Laid-open No. 05-043977 -   [Patent Literature 5] -   Japanese Patent Laid-open No. 2001-335884

SUMMARY OF INVENTION Technical Problem

However, in the chemical composition conventionally proposed and described in Patent Literature 1, since much of MA is produced in HAZ by multi-layer welding with the welding heat input being set at 1.6 kJ/mm or higher, the toughness is reduced and the CTOD characteristic cannot stably be secured.

In Patent Literature 2, rolling is needed in the manufacturing process and the manufacturing method of Patent Literature 2 cannot be applied to large structures containing thick-wall flanges of 150 mm or larger and the like.

By technologies proposed in Patent Literatures 3 and 4, it is very difficult to uniformly control micro-inclusions in thick-wall forged steels and the like in the steel manufacturing process thereof, and a stable effect cannot be attained.

Furthermore, in Patent Literature 5, since it uses a steel plate as its object, the content of Al and N is low; and in a thick-wall forged steel, the crystal grain diameter cannot be made fine during thermal refining, so the toughness of the member itself cannot be secured. Furthermore, although there is a prescription of adding B in the range of 0.0005 to 0.0015% for strength enhancement, it is difficult to make the B to be homogeneously contained in the whole in the thick-wall forged steel manufactured from a steel ingot. Besides, depending on heat handling during the thermal refining, borides detrimental to the toughness are precipitated. Therefore, in any one of the Patent Literatures, with regard to Cu-containing low alloy steels whose strength varies by the aging treatment of Cu, there are not made clear about the steels which are excellent in the toughness of multi-layer weld heat affected zones of thick-wall forged steels of 150 to 450 mm, and manufacturing methods thereof.

The present invention has been achieved with the above situation as the background, and has an object to provide a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and a manufacturing method thereof, in which the composition of the steel is optimized and the low-temperature toughness of the weld HAZ is enabled to be improved.

Solution to Problem

That is, among Cu-containing low alloy steels excellent in toughness of a weld heat affected zone according to the present invention, a first aspect thereof is a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the steel comprises a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.020 to 0.080% and N: 0.005 to 0.020%, the balance consisting of Fe and unavoidable impurities.

A second aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to the above aspect, wherein the chemical composition further contains, in % by mass, Ca: 0.010% or less.

A third aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to the above aspects, wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.

A fourth aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the above weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.

A fifth aspect of the present invention is the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FAIT) measured by a V-notch Charpy impact test of −70° C. or lower.

Among methods for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to the present invention, a first aspect thereof is a method comprising thermal refining conditions for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to any one of the above aspects, wherein the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in the temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in the temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C.

A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to another aspect of the present invention is a method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to any one of the above aspects, wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness.

A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to another aspect of the present invention comprises a step of the above thermal refining conditions in the above aspect.

Hereinafter, the composition and the conditions during manufacturing specified in the present invention will be described. Here, any of component contents in the composition is indicated in % by mass.

C: 0.01 to 0.06%

From the viewpoint of securing the strength, since C is a necessary element to be added, its lower limit is set at 0.01%. However, since the incorporation thereof exceeding 0.06% reduces the toughness of the weld heat affected zone, particularly the CTOD (Crack Tip Opening Displacement) characteristic, and reduces weldability, its upper limit is set at 0.06%. Then, for the same reasons, it is desirable that its lower limit is set at 0.02%, and its upper limit is set at 0.05%.

Si: 0.05 to 0.40%

Si is used as a deoxidizing element when melting and/or smelting alloys. Then since Si is a necessary element for securing the strength, its lower limit is set at 0.05%. However, since an excessive incorporation thereof increases the generating amount of MA formed of the weld heat affected zone and reduces the toughness, its upper limit is set at 0.40%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.10%, and its upper limit is set at 0.35%.

Mn: 0.20 to 0.70%

Mn is, similarly to Si, an element useful as a deoxidizing element, and since Mn also contributes to the improvement of hardenability, its lower limit is set at 0.20%. However, since an excessive incorporation thereof increases the generating amount of MA (Martensite Austenite) formed of the weld heat affected zone and reduces the toughness, its upper limit is set at 0.70%. Then, for the same reasons, it is desirable that its lower limit is set at 0.30%, and its upper limit is set at 0.60%, and it is more desirable that its upper limit is set at 0.50%.

Ni: 1.20 to 2.50%

Since Ni is a necessary element for securing the strength owing to an improvement in hardenability and securing the low-temperature toughness, its lower limit is set at 1.20%. However, since an excessive incorporation thereof stabilizes the retained γ and reduces the toughness, its upper limit is set at 2.50%. Then, for the same reasons, it is desirable that its lower limit is set at 1.50%, and its upper limit is set at 2.30%.

Cr: 0.50 to 1.00%

Since Cr is an important element for securing the hardenability and securing the strength and the toughness, its lower limit is set at 0.50%. However, an excessive incorporation thereof raises the hardenability, reduces the toughness and raises the weld cracking sensitivity, its upper limit is set at 1.00%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.60%, and its upper limit is set at 0.80%.

Cu: 0.80 to 1.50%

Cu is precipitated during aging treatment and improves the strength of the steel. In a low-carbon steel, it is very important to secure the strength owing to a Cu precipitate. Furthermore, since Cu is an important element also for improving the corrosion resistance, its lower limit is set at 0.80%. However, since an excessive incorporation thereof reduces the toughness and decreases the hot workability, its upper limit is set at 1.50%. Then, for the same reasons, it is desirable that its lower limit is set at 1.10%, and its upper limit is set at 1.30%.

Mo: 0.20 to 0.60%

Mo contributes to the improvement of the hardenability and is an important element for securing the strength and the toughness, its lower limit is set at 0.20%. However, since an excessive incorporation thereof reduces the toughness and decreases the weldability, its upper limit is set at 0.6%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.40%, and its upper limit is set at 0.50%.

Al: 0.010 to 0.050%

Al combines with N to form AlN, thereby suppressing crystal grain growth. Grain refining is essential for improving the toughness, and with regard to the content of Al, its lower limit is set at 0.010%. However, since an excessive incorporation thereof reduces the toughness due to coarse AlN, its upper limit is set at 0.050%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.015%, and its upper limit is set at 0.030%.

Nb: 0.020 to 0.080%

Since Nb is an important element for suppressing crystal grain growth as a carbonitride and refining the crystal grains, its lower limit is set at 0.020%. However, since an excessive addition thereof promotes aggregating and coarsening of the carbonitride and reduces the toughness, therefore its upper limit is set at 0.080%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.040%, and its upper limit is set at 0.050%.

N: 0.005 to 0.020%

N is an important element for suppressing crystal grain growth and refining the crystal grains as AlN and carbonitride, and its lower limit is set at 0.005%. However, since an excessive addition thereof promotes precipitating and aggregating and coarsening of a large amount of the AlN and carbonitride and reduces the toughness, its upper limit is set at 0.020%.

Then, for the same reasons, it is desirable that its lower limit is set at 0.006%, and its upper limit is set at 0.015%.

Ca: 0.010% or Less

Since Ca forms an oxide or a sulfide as Ca—Si, therefore, as desired, Ca is used as a deoxidizing and desulfurizing element. However, since an excessive addition thereof reduces the toughness, the addition is set at 0.010% or less. For the same reason, it is desirable that its upper limit is further set at 0.005%.

Area ratio of MA in ICCGHAZ: lower than 5% Area ratio of MA in the whole HAZ: lower than 4%

Since MA contains high-carbon martensite and retained γ, MA is very hard, and behaves as a hard phase. The presence of the hard phase makes a strength difference from a matrix phase and becomes a starting point of the stress concentration during brittle fracture. In particular, in a multi-layer weld HAZ (heat affected zone) formed at a multi-layer welded joint portion of a thick-wall material, since the region where a multiple welding heat cycle is imparted expands along with the increase of the welding heat input, the generating amount of MA is increased and the toughness, particularly the CTOD characteristic is reduced. For improving the toughness of the multi-layer weld HAZ, a reduction in the area ratio of MA is needed. The region in HAZ of the present kind of steel where the toughness is most reduced is ICCGHAZ (inter critically reheated CGHAZ, CGHAZ: coarse grain HAZ), and the area ratio of MA in this region lower than 5% and also the area ratio of MA in the whole HAZ lower than 4% are observed to provide an improvement in the toughness.

The area ratio of MA is, in the case of the quantity of heat input being 3.5 kJ/mm or lower, obtained from an average value thereof obtained in the whole HAZ or in the region of the ICCGHAZ. Here, the quantity of heat input is presented as a condition of the evaluation of characteristics, and the quantity of heat input during welding in the present invention is not limited to the above range.

0.2% yield strength after the thermal refining: 525 MPa or higher Ductile-brittle fracture appearance transition temperature (FAIT) after the thermal refining: −70° C. or lower

In a steel for offshore structures and the like which support riser pipe lines, since a tension is exerted on the members, the strength of the members themselves becomes needed. Therefore, by making the 0.2% yield strength after the thermal refining to be 525 MPa or higher, the steel becomes a steel having a sufficient strength even though used as the members of offshore structures on which a tension is exerted.

When offshore structures once cause large-scale destruction, the destruction has a tremendous influence on the environment. By setting the ductile-brittle fracture appearance transition temperature (FAIT) after the thermal refining to be −70° C. or lower, the destruction can be prevented.

Thermal Refining Conditions

In the case of quenching, the steel needs at least to be heated at a temperature of the Ac₃ transformation temperature or higher. Then, even when the heating temperature of quenching is the Ac₃ point or higher, since hardenability cannot be secured in the case where the temperature is low, the lower limit temperature is set at 850° C. However, since making the quenching temperature high coarsens the γ particle diameter during heating and reduces the toughness thereafter, its upper limit is set at 950° C. Here, the quenching can be repeated several times as required. Then, heating means and cooling means during the quenching are not especially limited, in the present invention, and means capable of providing desired heating capability and cooling capability can suitably be selected.

The quenched steel is then subjected to two phase region quenching treatment being heated in the temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower and thereafter cooled. Heating means and cooling means during the two phase region quenching are also not especially limited, in the present invention, and means capable of providing desired heating capability and cooling capability can suitably be selected. This heat treatment is the most important one in the manufacturing method according to the present invention.

The heating temperature in the above heat treatment is specified to the temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower. When the heating temperature is a temperature of lower than (Ac₃ transformation temperature−50° C.), since the amount of transformation to the γ phase is insufficient; the amount of the α phase subjected to the high-temperature tempering is large; and the Cu precipitate is coarsened, therefore the 0.2% yield strength cannot be secured. Furthermore, the crystal grains thereafter are also not refined; and the constituent concentration to the transformed γ phase is caused, and the γ phase remains even at room temperature, making it difficult for the toughness to be secured. Conversely, when the heating temperature is set at a high temperature exceeding (Ac₃ transformation temperature−10° C.), the amount of transformation to the γ phase becomes excessive, and since the transformed γ phase reduces hardenability and makes granular bainitic ferrite, a good metal structure cannot be obtained. Additionally, the crystal grain diameter becomes coarsened and a sufficient strength and the low-temperature toughness cannot be secured. For such reasons, the heating temperature is specified to the temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower.

Following the above two phase region quenching, the resultant steel is tempered in the temperature range of 500 to 600° C. When the heating temperature is lower than 500° C., the 0.2% yield strength is increased due to the aging effect of the Cu precipitate and the toughness is reduced. Then at a low tempering temperature, the internal stress during the thermal refining cannot be relaxed and damage in using of the steel is caused. Conversely, when the temperature exceeds 600° C., over-aging is made and the 0.2% yield strength cannot be secured. Therefore, the temperature range of the tempering treatment is set at 500 to 600° C.

Thick-Wall Part

The present invention can suitably be applied to manufacture of materials having thick-wall parts. There is exemplified a material whose thick-wall part has a maximum wall thickness of 150 mm or larger and 450 mm or smaller.

In a material having a wall thickness of 150 mm or larger, the temper rolling is difficult, and the advantageous effects of the present invention can remarkably be attained. On the other hand, when the wall thickness exceeds 450 mm, in the cooling process of quenching and two phase region quenching, the cooling rate is decreased and the strength is decreased.

Advantageous Effects of Invention

That is, according to the Cu-containing low alloy steel according to the present invention, an excellent low-temperature toughness can be secured by optimizing the composition.

According to the method for manufacturing the Cu-containing low alloy steel according to the present invention, by specification of the compositional range and thermal refining and the like, it is enabled the manufacture of a thick-wall Cu-containing low alloy forged steel which is suitable as a steel for offshore structures used for mooring facilities, risers, flowlines and the like, and is excellent in the low-temperature toughness, and particularly has a high strength and high toughness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic explanatory diagram showing an example of a quenching method of the present invention.

FIG. 2 is a schematic diagram of a weld heat affected zone of a multi-layer welded joint.

FIG. 3 is photographs as a substitute of drawings showing SEM images after Le Pera etching of an Invention Example and a Comparative Example.

FIG. 4 is a graph showing relations between the CTOD value of welded joints and the notch position of an Invention Example and a Comparative Example.

DESCRIPTION OF EMBODIMENTS

A Cu-containing low alloy steel to be used in the present invention, if a composition specified by the present invention is set to the target, can be ingoted by a common method, and in the present invention, the method is not particularly limited.

The manufactured steel ingot is hot forged into an arbitrary shape, and thereafter, subjected to the above-mentioned quenching (Q), two phase region quenching (L) and tempering (T) treatment.

Here, the content and a method of the hot forging are not especially limited, and the forging ratio and the like are also not especially limited. A material can be hot forged into one having a thick wall, for example, into a material having a thick-wall part of 150 mm to 450 mm in wall thickness. Although the Cu-containing low alloy steel according to the present invention brings about an especially suitable effect on a material applied as a steel for the above-mentioned large structure having a thick-wall part, the present invention is not especially limited in the wall thickness, and can also be used in applications whose thickness is smaller than the above.

In the thermal refining treatment, the Cu-containing low alloy steel is quenched by being heated in the temperature range of 850 to 950° C. Thereafter, the quenched steel is two phase region quenched in the temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower, and furthermore tempered at 500 to 600° C.

Then, between the hot forging and the thermal refining treatment, a heat treatment such as normalizing (N) can also be carried out. As this normalizing condition, the heating condition of 950 to 1,000° C. can be exemplified.

Furthermore, between the hot forging and the quenching treatment, a heat treatment such as normalizing (N) can also be carried out. As this normalizing condition, the heating condition of 950 to 1,000° C. can be exemplified.

A heating pattern of the above thermal refining conditions is shown in FIG. 1.

The steel is, in the first quenching, heated at a temperature of the Ac₃ or higher, and heat treatment is performed so that the heating temperature during the two phase region quenching falls in the specified range. Then in the tempering treatment, the steel is heated at a temperature of the Ac₁ or lower for heat treatment.

According to the specification of the compositional range and the manufacturing method thereof according to the above, it is enabled the manufacture of a thick-wall Cu-containing low alloy forged steel which is suitable as a steel for offshore structures used for mooring facilities, risers, flowlines and the like, and is excellent in the low-temperature toughness, particularly in the balance between the strength and the low-temperature toughness.

The Cu-containing low alloy steel obtained in the above has characteristics of a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of −70° C. or lower.

Here, in the present invention, a welding method to be applied to the formation of a multi-layer welded joint is not especially limited.

In the present embodiment, a structure of the weld heat affected zone (HAZ) after affected by heat at a quantity of heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.

Furthermore, a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the above weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.

Here, the weld heat affected zone is defined as a region where a microstructure or macrostructure of a base metal has been changed due to the welding heat effect.

For example, in a macrostructure observation, a weld cross section is etched by using a mixed aqueous solution of ammonium copper chloride and hydrochloric acid, a 10% nital or the like; and in a microstructure observation, a weld cross section is etched by using a 2% nital, a 5%-nital, a 5%-nitric acid alcohol solution, a mixed alcohol solution of a 5%-nitric acid and a 1%-picric acid, 5%-picral or the like.

A distinction can be made, visually or by using a microscope, between the weld heat affected zone and the base metal from changes in the structure emerging by the etching. Here, an etchant used for the etching is not limited to the above as long as being one capable of observing differences in the structure.

Example 1

Hereinafter, Examples of the present invention will be described comparing with Comparative Examples. Test materials having compositions indicated in Table 1 were ingoted into 50 kg steel ingots by a vacuum induction melting furnace. The ingoted steel ingots were each hot forged at 1,250° C. into 45 mm in thickness×130 mm in width (forging ratio: 3.1 s or more), and furthermore subjected to N treatment (960° C.), thereafter, subjected to Q treatment (900° C.), L treatment (800° C.) and T treatment (580° C.)

Here, cooling of the Q treatment and the L treatment was set at a cooling rate (20° C./min), which equivalently simulated that of water cooling of a plate thickness of 350 mm. Thereafter, a heat cycle for reproducing a multi-layer weld heat affected zone was imparted.

In a HAZ structure reproduced in the present Example, a coarse grain region of the first pass, which is the region that the toughness was most reduced in the present steel, was made into an ICCG (Inter-Critically Coarse Grain) HAZ two phase region reheated in the second and subsequent passes, as shown in FIG. 2. Furthermore, a structure of a CG (Coarse Grain) HAZ, which was a coarse grain region of the first pass, was also reproduced.

By imparting a reproducing heat cycle in simulation of CGHAZ and ICCGHAZ structure of a multi-layer weld heat affected zone with a quantity of heat input of 3.5 kJ/mm, the area ratios of MA of the CGHAZ and ICCHAZ and the toughness of the ICCGHAZ were accurately evaluated.

The quantity of heat input was obtained by the following expression and was determined at the conditions providing a quantity of heat input of 3.5 kJ/mm.

Quantity of heat input=60×the current(A)×the voltage(V)/the welding rate(mm/min)

Materials of 15×15×65 mm in size were sampled from the above forged plates after the thermal refining, and were subjected to a reproducing heat cycle test. The CGHAZ structure was prepared in simulation of a structure of the coarse grain region in the vicinity of a welding line of the first pass being treated at a maximum heating temperature of 1,350° C. for a holding time of 5 sec. Furthermore, after the preparation of the CGHAZ structure, the ICCGHAZ structure was prepared in simulation under the conditions of two phase region reheating of the second pass being carried out at a maximum heating temperature of 780° C. for a holding time of 5 sec. In each of reproducing heat cycles, in order to simulate the heat history of the multi-layer welding with a quantity of heat input of 3.5 kJ/mm, the temperature rise rate was set at 70° C./sec; and the cooling time in the range of 800° C. to 500° C. was set to 50 sec.

Here, heating in the heat cycle was carried out by high-frequency heating; and cooling was carried out by spraying of carbon dioxide gas or helium gas, and the temperature rise, the temperature holding and the cooling were controlled based on a measurement value of a couple attached on the test piece surface. Furthermore, in Steel A and Steel H, a test plate imparted with the thermal refining (Q-L-T) in simulation of a plate thickness being 350 mm was used, and subjected to submerged arc welding on a J groove with a heat input of 3.5 kJ/mm, and subjected to a CTOD test in which notches were formed at the fusion line (F.L.) of the weld metal, at 1 mm (F.L.+1 mm) on the HAZ side from the F.L. or at 2 mm (F.L.+2 mm) on the HAZ side from the F.L., and a CTOD test was carried out on the resultant.

TABLE 1 A_(C3) Chemical composition (% by mass) Transformation Steel N temperature type G Si Mn Ni Cr Cu Mo Al Nb Ca (ppm) (° C.) Remarks A 0.03 0.35 0.42 2.15 0.70 1.25 0.45 0.024 0.043 0.001 70 845 Invention B 0.03 0.10 0.41 2.14 0.71 1.24 0.44 0.020 0.044 0.001 81 845 Steel C 0.03 0.10 0.60 2.13 0.71 1.23 0.45 0.023 0.043 0.001 80 840 D 0.05 0.35 0.42 2.15 0.70 1.25 0.45 0.024 0.043 <0.001 88 830 E 0.03 0.35 0.40 0.95 0.70 1.23 0.45 0.022 0.045 0.001 72 850 Comparative F 0.03 0.35 0.80 2.15 0.72 1.24 0.45 0.025 0.046 <0.001 95 825 Steel G 0.03 0.35 1.40 2.15 0.71 0.61 0.44 0.023 0.045 <0.001 86 820 H 0.03 0.25 1.42 2.14 0.72 1.24 0.45 0.026 0.047 0.001 82 815

Test pieces were sampled from the reproducing heat cycle test pieces of the CGHAZ and the ICCGHAZ, and the area ratios of MA were measured. Furthermore, a CTOD test was carried out for the reproducing heat cycle test pieces of the ICCGHAZ to evaluate the low-temperature toughness. The test method was as follows.

The measurement of the area ratio of Ma involved making MA to appear by the Le Pera etching method, thereafter tracing five photographs of 2,500 times of an electron microscope (SEM), carrying out image analysis respectively and calculating an average value thereof.

The CTOD test involved sampling a test piece of 10 mm×10 mm×46 mm from the reproducing heat cycle test piece, and being carried out according to the specification of ISO12135. The test temperature was set at −20° C. and the number of the test pieces tested was three; and the low-temperature toughness was evaluated by using a critical CTOD value which was the lowest CTOD value among the obtained results.

Furthermore the fracture morphology was also evaluated from a stable crack extension amount. The fracture morphology is indicated by parentheses in Table, and δm indicates that no stable crack extension occurred until a maximum load point; and δc indicates that the stable crack extension occurred in 0.2 mm or less and then, unstable crack extension occurred. Furthermore in the test for welded joints of Steel A and Steel H, a test piece of 15 mm×30 mm×138 mm was sampled and the test was carried out according to the specification of ISO15653. The test temperature was set at −20° C. and the number of the test pieces tested was three.

It is shown in Table 2 the results obtained by using the reproducing heat cycle test pieces. It is shown in FIG. 3 the SEM images after the Le Pera etching of the reproduced ICCGHAZ structures of Steel 1 and Steel 8.

TABLE 2 Critical CTOD of Steel Steel type Area ratio of MA (%) ICCGHAZ No. (Table 1) CGHAZ ICCGHAZ (mm) Remarks 1 A 2.4 3.5 0.12(δm) Invention 2 B 2.4 3.1 0.15(δm) material 3 C 2.5 4.0 0.12(δm) 4 D 3.7 4.5 0.16(δm) 5 E — 3.1 0.03(δc)  Comparative 6 F 5.5 7.8 0.01(δc)  material 7 G — 7.8 0.01(δc)  8 H 6.7 8.9 0.01(δc) 

As is clear from Table 2, in Invention Steels of Steels Nos. 1 to 4, the CGHAZ and the ICCGHAZ both had a low area ratio of MA and the Invention Steels had a good toughness of ICCGHAZ. Steel No. 5, though having a low area ratio of MA, had a low toughness of HAZ. This is because since the Ni content deviated from the lower limit of the Ni content of the present invention, the hardenability was low and a structure having a low toughness of HAZ was formed. Furthermore, in Steels Nos. 6 to 8, the area ratio of MA of the ICCGHAZ was 5% or higher; and in Steels Nos. 6 and 8, the area ratio of MA of the CGHAZ was 4% or higher. Consequently, the HAZ toughness was remarkably low as compared with the present invention Steels.

As shown in FIG. 3, in Steel No. 1, which was an Invention Steel, a little of MA (white phase in figure) was observed. By contrast, in Steel No. 8, which was a Comparative Steel, much of MA was generated, and furthermore the shape thereof was long and narrow. By meeting the requirement of the present invention, the generating amount of MA could be suppressed and consequently, a good HAZ toughness could be provided.

Then it is shown in FIG. 4 the results obtained from the CTOD test of welded joints of Steel A and Steel H.

In Steel A, which met the requirement of the present invention, it is clear that good toughness was attained in the HAZ in a F.L. vicinity and at F.L.+1 mm.

By contrast, in the case of Steel H being a Comparative Steel, the toughness in the case of notched at positions of a F.L. vicinity and F.L.+1 mm was remarkably low.

From the above-mentioned results, for Steels A to D, having chemical compositions meeting the requirement of the present invention, it is possible to manufacture the Cu-containing low alloy steels having excellent toughness in HAZ having been imparted with the heat cycle of multi-layer welding.

The optimal thermal refining conditions capable of providing characteristics as thick-wall forged steels of steels for offshore structures was inspected by using Invention Steels A to D. Each of the materials was ingoted, forged, subjected to N (960° C.) and Q (900° C.), and thereafter thermally refined under the thermal refining conditions indicated in Table 3. Here, the Q treatment of Examples was all carried out at a temperature of 900° C., but for the above-mentioned reason, when the quenching temperature was in the range of 850 to 950° C., the Q treatment temperature was not especially limited. Then the cooling of the Q treatment and the L treatment was set at a cooling rate (20° C./min), which equivalently simulated that of water cooling of a plate thickness of 350 mm.

Test pieces were sampled from the obtained test materials, and were subjected to a tensile test and a Charpy impact test and evaluated for the strength and the low-temperature toughness. Test methods were as follows.

TABLE 3 Steel Heat treatment condition Steel Type Heat L temperature T temperature No. (Table 1) treatment (A_(C3)-50)° C. (A_(C3)-10)° C. (° C.) (° C.) Remarks 9 A QLT 795 835 820 520 present Invention Example 10 A QLT 795 835 820 580 present Invention Example 11 A QLT 795 835 820 450 Comparative Example 12 A QLT 795 835 780 550 Comparative Example 13 A QLT 795 835 780 450 Comparative Example 14 B QLT 795 835 820 550 present Invention Example 15 B QLT 795 835 820 580 present Invention Example 16 C QLT 790 830 800 580 present Invention Example 17 C QLT 790 830 815 580 present Invention Example 18 D QLT 780 820 780 600 present invention Example

In the tensile test, round bar tensile test pieces (parallel part diameter: 12.5 mm, G.L.: 50 mm) were sampled from the obtained test materials; and the tensile test was carried out at room temperature according to the specification of JIS 22241 and the 0.2% yield strength (Y.S.) and the tensile strength (T.S.) were determined.

In the impact test, 2 mm-V notch Charpy impact test pieces were sampled from the obtained test materials, and the impact test was carried out according to the specification of JIS 22242. The FATT was taken from a transition curve obtained by carrying out the Charpy impact test at optional temperatures. The obtained results are shown in Table 4.

TABLE 4 Steel Tensile properties Toughness Steel Type 0.2% Y.S. T.S. FATT No. (Table 1) (MPa) (MPa) (° C.) Remarks 9 A 620 732 −75 present Invention Example 10 A 542 642 −84 present Invention Example 11 A 712 857 −30 Comparative Example 12 A 510 621 −65 Comparative Example 13 A 640 752 −32 Comparative Example 14 B 572 689 −71 present Invention Example 15 B 538 654 −83 present Invention Example 16 C 538 656 −79 present Invention Example 17 C 567 683 −82 present Invention Example 18 D 610 741 −87 present Invention Example

Steels No. 9 (present Invention Example) and No. 10 (present Invention Example) had been subjected to L and T under the thermal refining conditions according to the present invention, in each of cases, good results in both of the 0.2% yield strength and the low-temperature toughness were obtained.

By contrast, as for Steels Nos. 11 to 13 (Comparative Examples), though using Steel A, which was the same steel kind as Steels Nos. 9 and 10, exhibited a reduced toughness or strength as compared with the Invention Examples, because the conditions used for the heat treatment process was out of the requirement of the present invention. As for Steel No. 11, since having undergone a low T temperature, age hardening of Cu becomes excessive and no good toughness is obtained. Steel No. 12 underwent a low L temperature and though having undergone a treatment under a suitable T temperature, was reduced in both of the strength and the toughness as compared with the Invention Examples. Furthermore Steel No. 13 underwent a low T temperature treatment in order to improve the strength of Steel No. 12, and was increased in the strength, but was reduced in the toughness correspondingly. Therefore, it is clear that only in the case where the temperatures of L and T of the thermal refining conditions were suitable, a good strength and toughness could be attained.

Results of Steels Nos. 14 and 15 were those using Steel B. Each of the steels, since meeting the requirement of the present invention, gave a good strength and toughness.

Results of Steels Nos. 16 and 17 were those using Steel C. Each of the steels, since meeting the requirement of the present invention, gave a good strength and toughness.

Result of Steel No. 18 was those using Steel D. The present steel, since meeting the requirement of the present invention, also gave a good strength and toughness.

From the above-mentioned results, by application of the proper chemical composition and manufacturing process, it becomes possible to manufacture, as a thick-wall forged steel product, the Cu-containing low alloy steel having an excellent low-temperature toughness of the multi-layer weld heat affected zone and a good strength and toughness of the member itself.

Hitherto, the present invention has been described based on the above-mentioned embodiments and the above-mentioned Examples, but the technical scope of the present invention is not any more limited to the contents of the above description, and the contents of the above-mentioned embodiments may suitably be changed and modified without departing from the scope of the present invention. 

1. A Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the steel comprises a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.020 to 0.080% and N: 0.005 to 0.020%, the balance consisting of Fe and unavoidable impurities.
 2. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein the chemical composition further contains, in % by mass, Ca: 0.010% or less.
 3. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.
 4. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.
 5. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of −70° C. or lower.
 6. A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein the method comprises thermal refining conditions for manufacturing the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature—10° C.) or lower, and further tempering the resultant at 500 to 600° C.
 7. A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 1, wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness.
 8. The method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 7, the method comprising a step of a thermal refining conditions, the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C.
 9. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 2, wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%.
 10. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 2, wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%.
 11. The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 2, wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of −70° C. or lower.
 12. A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 2, wherein the method comprises thermal refining conditions for manufacturing the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C.
 13. A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 2, wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness.
 14. The method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to claim 13, the method comprising a step of a thermal refining conditions, the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac₃ transformation temperature−50° C.) or higher to the (Ac₃ transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C. 